Waste heat recovery systems can make available for use energy in exhaust gases and other heat sources that would otherwise be lost. When incorporated in a vehicle with an internal combustion engine, waste heat recovery systems add certain advantages. For example, and not limitation, the waste heat recovery system call be designed to recover heat from exhaust gas or the EGR (exhaust gas recirculation) system, which reduces the cooling load on the engine cooling system. In addition, a waste heat recovery system can extract useful energy from the exhaust gas exiting the tail pipe or exhaust stack, which would otherwise be lost to the environment.
The amount of waste heat recovered can vary according to a number of conditions, including, for example, engine load and engine running time.
By way of example, shortly after start up or during low RPM operation less waste heat may be available for recovery than after a vehicle has warmed up or during intermediate to high RPM operation. Those of ordinary skill in the art will appreciate that a working fluid used in WHR must be heated to a minimal threshold before useful energy can be efficiently generated from the working fluid. A system designed only to efficiently make available energy during intermediate or high engine load or wasted heat operation or after a vehicle has warmed up may not be very efficient at making energy available during low engine load operation or shortly after start up.
By way of another example, after a vehicle has warmed up and during intermediate to high engine load operation a large amount of waste heat may available for recovery than shortly after start up or during low engine load operation. Those of ordinary skill in the art will appreciate that after the working fluid is heated and used make available energy in WHR that it must often times be cooled and condensed before being able to be reheated to make available more energy. A system designed only most efficiently make available energy shortly after start up or during low engine load operation may heat the working fluid to an excessive degree during high engine load operation and thus increase the duration of cooling and condensing cycle without increasing the amount of energy made available.
The present invention provides a method and apparatus for improved flexibility in the recovery of waste heat from the working fluid of a WHR.